The present invention relates to a leakage flux probe for non-destructive leakage flux-testing of bodies consisting of magnetisable material, in particular of pipes consisting of ferromagnetic steel, having a plurality of coils for detection of near-surface flaws in the body, wherein the coils have a degree of sensitivity which is dependent upon the orientation of the flaws in the body.
For the purpose of non-destructive, near-surface testing of bodies consisting of magnetisable materials, it is generally known to use a so-called leakage flux method. For this purpose, the bodies which are to be tested are magnetised temporarily by electromagnets, cylinder coils or current linkage. In an homogeneous and flawless ferromagnetic material, the magnetic field lines are distributed uniformly over the surface. If the homogeneity of the material is disrupted by near-surface discontinuities, such as e.g. cracks, cavities, inclusions, pores or laminations, then magnetic field lines can emerge as so-called leakage flux from the workpiece in the region of the discontinuities. This leakage flux can be detected in a contacting or contactless manner by probes. A corresponding testing device typically includes a magnetisation unit, a handling unit for the body, a testing shoe having the leakage flux probes, an evaluating unit and optionally a demagnetisation unit. Current flux probes used for measuring the magnetic leakage flux density include induction probes, Giant-Magneto-Resistance probes (GMR-probes) or Hall-probes.
This known leakage flux-testing is also applied e.g. in the case of pipes consisting of ferromagnetic steel, in order to detect longitudinally and transversely oriented, as seen in the longitudinal direction of the pipes, and near-surface discontinuities on the inner and outer surfaces. In this case, unidirectional field magnetisation of the pipe is typically used, since flaws on the outer surface and on the inner surface of the pipe can be detected thereby. Alternating field magnetisation, which is used e.g. in the case of bar stock, can only detect flaws on the outer surface. In order to test the pipe for longitudinal flaws a magnetic field is applied at right angles and for transverse flaw testing a magnetic field is applied in parallel with the longitudinal axis of the pipe. In order to detect the entire surface when testing for longitudinal flaws in the pipe, the pipe and the probe are moved in helical fashion with respect to each other. Typically, when testing for transverse flaws, a probe having a probe ring is fixedly positioned around the pipe and serves to then move the pipe in the longitudinal direction. In order to calibrate the testing device, one or several grooves introduced onto a reference workpiece are used as a test flaw reference. The grooves simulate longitudinal, oblique and transverse flaws.
The German patent specification DE 198 23 453 C2 already discloses a leakage flux probe for non-destructive testing of elongate and rotationally symmetrical bodies, in particular pipes, for longitudinal or transverse flaws. The leakage flux probe consists substantially of a ruler-shaped printed circuit board, on whose side facing the body to be tested a plurality of coil pairs are printed. A total of 16 coil pairs are provided which as seen in the longitudinal direction of the printed circuit board are disposed in succession at a respectively identical spaced interval. Each individual coil of a coil pair comprises an elongate, substantially running track-like winding. Each winding is ring-shaped having a central longitudinal axis. The coils of a coil pair are each disposed slightly obliquely in relation to the longitudinal direction of the printed circuit board, so that in each case the longitudinal axis of the coils and the longitudinal direction of the printed circuit board form approximately an angle of 10 degrees. Moreover, as seen in the longitudinal direction of the printed circuit board, both coils or a pair are disposed laterally next to each other at a spaced interval and are offset with respect to each other in the longitudinal direction of the printed circuit board, so that as seen in the longitudinal direction of the printed circuit board the right-hand coil of a pair protrudes approximately two thirds of the length of the coil with respect to the left-hand coil. In this case, the coils of a pair are inclined to the right.
With the known leakage flux-testing, two mutually separate testing devices are used to reliably identify any longitudinal flaws in a first test and transverse flaws in a second test. Longitudinal and transverse flaw testing only identifies to a limited extent any oblique flaws extending obliquely with respect to the magnetic field direction.
Furthermore a sensor unit for magnetic testing of cracks is known from the Japanese laid-open document JP 59 202 057 A. The sensor unit consists of four sensors which are disposed one behind the other in series and are electrically connected in series. Each of the sensors is constructed in a manner comparable to a GMR-probe and their detection system is based upon magnetic resistance change. Moreover, each sensor consists of two first and second sensor parts which are disposed one behind the other in series and formed in each case in a meandering or comb-like manner. In this case, the tines of the comb of the first sensor part extend at a right angle to those of the second sensor part. Electrical contacts are provided between the first and second sensor parts and at the beginning and end of the sensor unit. This sensor unit is said to have the advantage that an adjustment of the amplification of the sensitivity for each sensor is to be avoided.
Furthermore, a further sensor unit for mechanical testing of cracks is known from the Japanese laid-open document JP 07 294 490 A, by means of which cracks extending in an oblique manner are to be identified in a particularly effective manner. For this purpose, a total of 16 sensors are provided which are disposed on a ruler-shaped holding element. In this case, two sensors in the form of a pair are each disposed next to each other and slightly offset with respect to each other as seen in the longitudinal direction of the holding element. Of these pairs, a total of 8 are disposed one behind the other as seen in the longitudinal direction of the holding element, which means that the arrangement of the sensors on the holding element can be described as being zigzag-like on the whole. In order to identify cracks, two of the sensors are each operated in a differential circuit. If a negatively oblique direction of the cracks is identified, two sensors which are disposed next to each other as seen in the longitudinal direction of the holding element are connected together in a differential circuit. In the event that a positively oblique direction of the cracks is detected, two sensors which are disposed one behind the other and next to each other as seen in the longitudinal direction of the holding element are connected together in a differential circuit. By virtue of this variable differential circuit, different sensors can be connected together in a differential circuit in dependence upon an orientation of a detected crack.